Due to the scarcity of radio spectrum resources and the demand for high speed data transmission, wireless communication systems are required to achieve higher spectral efficiency as well as higher power and cost efficiencies. With the advance of digital signal processing and radio technologies, multiband systems have become more and more appealing since they can accommodate wider spectrum for achieving higher data rates and provide more flexible and adaptive use of the existing frequency bands.
Orthogonal frequency division multiplexing (OFDM) has been widely used in recent times in various single band communications systems, such as the wireless local area networks (WLANs) and the 3rd Generation Partnership Project (3GPP; www.3gpp.org) Long Term Evolution (LTE) systems. OFDM is characterised by adjacent subchannels within a band exhibiting orthogonality. OFDM is also a suitable modulation technique for use in multiband systems due to the flexibility for subcarrier allocation and the implementation simplicity with fast Fourier transform (FFT) as a frequency band can be dynamically selected or de-selected by turning on or off the subcarriers falling in the band according to the band assignment. However, OFDM exhibits out-of-band emission due to the slow side lobe roll-off of the subcarriers, which can cause inter-channel interference without further reduction to meet the transmit mask requirement.
There are existing techniques for out-of-band emission reduction in OFDM-based multiband systems. The first straightforward technique is to apply notch filters to the unallocated bands. However, a digital implementation of this filter would increase the processing complexity considerably, and an analogue implementation would be costly and difficult to achieve dynamic band allocation.
The second technique is to introduce guard bands on the edges of the transmitted signal bands. Unfortunately, this will sacrifice spectral efficiency and may not be able to provide sufficient protection without having significantly large number of subcarriers used for an allocated frequency band.
The third technique is to perfoun windowing to the transmitted signal in the time-domain. This requires an extended OFDM symbol with extra signal power and causes inter-symbol interference. Guard bands may also need to be used together with windowing to ensure satisfactory out-of-band emission reduction. (See, for example, IEEE Standard 802.11a-1999, “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: High-speed Physical Layer in the 5 GHz Band”.)
The fourth technique is to compose interference cancellation subcarriers and place them on the edges of the transmitted signal bands. This technique not only reduces the spectral efficiency but also degrades the effective signal-to-noise ratio (SNR) at the receiver since the extra signal power for the cancellation is wasted. (See, for example, S. Brandes, I. Cosovic, and M. Schnell, “Reduction of Out-of-Band Radiation in OFDM Systems by Insertion of Cancellation Carriers,” IEEE Communications Letters, Vol. 10, No. 6, June 2006, pp. 420-422.)
There is therefore a need for out-of-band emission cancellation techniques which can substantially avoid the problems of the known approaches.